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Modbus Commands

2-6

SIMATIC TI505/TI500 MODNIM User Manual

2.3 Comparing Modbus Functions with TIWAY I Network Commands

This

section outlines some of the differences between Modbus and

TIWAY

I network commands.

•

Whereas Modbus uses

Functions Codes, TIW

Y I uses

Primitives.

Primitives differ from Function codes in one important way: primitives

use TT

-types to allow one primitive to address different types of data.

ith Modbus a different function code is required for each type of data.

• W

ith SIMA

TIC



PLCs, the first address for each type of memory is

always 1. By comparison, Modbus PLCs allow 0 as the first legal

address.

•

If each system were to return one byte (8 bits) representing eight

discrete coils, they would each be positioned as shown in Figure 2-3.

8 7654321 87654321

Coil

No.

Coil No.

Modbus PLCs

SIMA

TIC TI PLCs via MODNIM

SIMATIC TI PLCs

via TIWAY

MSB LSB<

< <

MSB LSB

< < <

Figure 2-3 Bit

Orientation

• T

erminology is another area in which differences occur

. Some of the

differences in terms are given in T

able 2-3. (Refer also to Appendix C,

able C-3 for a list of the TIW

Y primitive equivalents to the Modbus

function codes.)

able 2-3

Ter

minology Dif

ferences

Modbus-based PLCs SIMATIC TI PLCs

Coil Discrete output (Y) or Control Relay (C)

Input Register Word Image Register (WX/WY)

Holding Register Variable Memory (V-memory)

Slave Secondary Node

P/C (Programmable Controller) PLC

Function code Primitive

Modbus TIWAY I

Disabled (coil) Forced

Enabled (coil) Unforced

Modbus

Functions

vs. TIW

Commands

1 2 ... 32 33 34 35 36 37 38 39 40 41 42 ... 90 91

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Siemens SIMATIC TI505 Manual de usuario Pagina 37

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